This invention relates to a phenolic polymer useful as an antioxidant and a process for producing the same.
As antioxidants, there has widely been used 2,6-di-tertiary-butyl paracresol as a phenolic compound. But since it has a boiling point of 265.degree. C., it volatilizes under kneading conditions (270.degree.-350.degree. C.) with a thermoplastic resin such as polypropylene. In order to prevent this, pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and the like are used as an antioxidant for thermoplastic resins. Recently, with an increase of processing temperature of thermoplastic resins, for example, up to 350.degree. C. in the case of engineering plastics, even the latter antioxidant volatilizes. For this reason, heat resistant phenolic antioxidants have been desired.
It is proposed in U.S. Pat. No. 3,004,953 to produce a special phenolic resin suitable as an antioxidant by reacting a phenol with para- or metadiisopropenylbenzene using an alkylation catalyst such as Bronsted acid or Lewis acid at 60.degree. to 120.degree. C. The molecular weight, structure and repeating units of the special phenolic resin are not disclosed therein, but when the phenol is used in excess in molar ratio to the para- or meta-diisopropenylbenzene, the phenol moiety is present at both ends of the resulting polymer. When used as an antioxidant, it is necessary to have substituents at both ortho positions of the phenol. When hydrogen is present at the ortho positions, the effect is reduced.
According to experiments of the present inventors after the process of said U.S. Pat. No. 3,004,593 using para-cresol and para-diisopropenylbenzene, it was found by gel permeation chromatography that a large amount of homopolymer of para-diisopropenylbenzene is produced in addition to an alkylation reaction product of para-diisopropenylbenzene with para-cresol, and at the same time a large amount of para-cresol is retained uncreacted. This means that the polymer obtained by the process of U.S. Pat. No. 3,004,953 is a mixture of the alkylation reaction product and the homopolymer, and the amount of phenolic hydroxyl group per unit weight of the obtained resin is smaller than that obtained from calculation considering the amounts of the starting compounds.
Therefore, when the special phenolic resin of said U.S. patent is used as an antioxidant, a large amount of it should disadvantageously be added to thermoplastic resins in order to exhibit its effect.
On the other hand, it is proposed in U.S. Pat. No. 3,996,198 to produce a phenolic polymer by reacting a phenolic compound having hydrogens at both ortho positions with respect to a phenolic hydroxyl group with a diolefin compound by using a metallic aluminum to carry out the ortho-alkylation reaction. But since the ortho positions of terminal phenolic compound moieties of the resulting phenolic resin are not sealed, the effect as an antioxidant is little.
Further, it is also known crystalline cresol derivatives obtained by dehydration condensation of two molecules of cresol and one molecule of bis(1-hydroxy-1-methylethyl)benzene (Japanese Patent Unexamined Publication No. 58-121231) and crystalline alkyl-disubstituted phenol derivatives obtained by dehydration condensation of two molecules of alkyl-disubstituted phenol and one molecule of bis(1-hydroxy-1-methylethyl)benzene (Japanese Patent Unexamined Publication No. 58-121230). Since these cresol derivatives and alkyl-disubstituted phenol derivatives are crystalline monomeric compounds, there is a fundamental defect in that almost of these compounds volatilize under the kneading conditions with thermoplastic resins (270.degree. to 350.degree. C.).